The prior art methods of measuring the capacitance of large capacitors having an appreciable amount of ESR (equivalent series resistance) have been relatively time consuming, requiring several minutes for each measurement. Attempts to speed up such tests have introduced appreciable errors, primarily because of the ESR.
One such prior art method requires the measurement of the reactance of the capacitor while a signal having a predetermined frequency is applied to the capacitor. When the capacitance is high, the capacitive reactance is very low, usually less than one ohm, wherefor it is very difficult to make an accurate measurement of the reactance. Furthermore, even relatively low values of ESR make the accurate measurement of the capacitive reactance practically impossible.
In a second such method called the RC test, the capacitor under test is charged to a predetermined voltage level through a resistor having a known resistance. The time required to charge the capacitor to the predetermined level is measured and used to calculate the value of capacitance. Since the time required to charge the capacitor is proportional to the capacitance, it takes a long time to measure the capacitance of a large capacitor. This test can be speeded up by reducing the resistance of the series connected resistor through which the capacitor is charged, but as that resistance is lowered the effect of ESR increases and reduces the accuracy of the measurement.
Another commonly used method is known as the constant current test. When using this method the capacitor is connected across a constant current source and the time required to charge the capacitor between two different voltage levels is measured and used to calculate the capacitance. Where ESR is present and the capacitance is high, the time required to accurately measure the capacitance is necessarily long since the charging current must be kept relatively low to minimize the effect of ESR.
Recently a new type of capacitor having very high capacitance and very high ESR has become commercially available. Because of its construction it is known as the double layer capacitor. Testing these capacitors with known test methods has exacerbated the problems associated with time and accuracy. For example, when using the RC test to measure a capacitance of 100 .mu.F., it takes 100 milliseconds to charge the capacitor to sixty-three percent of its full charge through a 1000 ohm resistor Using the same 1000 ohm series resistor to test a ten farad capacitor, it would take 166.6 minutes or 2.7 hours to make the same test. It would seem logical to reduce this time by lowering the resistance value. However, if the time is to be appreciably reduced in this manner a falsely low capacitance value will result where the capacitor has even a normal amount of ESR let alone the high values associated with the new double layer capacitors. Use of the constant current test provides even more erroneous measurements because increasing the charging current in order to decrease the time of the test magnifies the effect of ESR. As a consequence, use of the prior art methods of measuring capacitance requires long periods of time unless large error factors can be tolerated.